The invention relates to a method as well as to an arrangement for clipping a complex baseband input signal, especially baseband input signals of a multicarrier transmission system.
High-quality modulation methods are used with mobile communication systems, in which symbols with more than two possible information states are transmitted.
These modulation methods for example include the known modulation methods QPSK, 8 PSK and 16 QAM, in which, to obtain a higher data transmission rate, a constant envelope end of an instantaneous transmit power of a carrier-frequency transmission signal is dispensed with. This means that an instantaneous transmit power varies over time.
A variation of the instantaneous transmit power is also known in multicarrier transmission systems, in which signals for example are transmitted by OFDM radio transmission using subcarriers.
Depending on the modulation method used in each case and the combination of subcarriers used, an instantaneous maximum power of a transmit signal considered can lie far above an average value of the transmit power. The ratio of the maximum transmit power to average transmit power is referred to as the “peak to average ratio, PAR” or as the “peak to average power ratio, PAPR” whereby in the above case, the following applies: PAR>1 or PAPR >1.
With a PAR value of this type what is known as a control reserve is to be retained on the transmit amplifier side used, to avoid non-linear distortions of the transmit signal by the transmit amplifier. The need to do this increases the complexity and the power consumption of the transmit amplifier and simultaneously reduces efficiency of an overall transmission system considered.
Clipping methods are known with the aid of which the influence of a non-linear characteristic curve of the transmit amplifier on an amplified carrier-frequency transmit signal can to a certain extent be compensated for.
By applying such clipping methods it is possible to minimize the control reserve to be preserved on the transmit amplifier side.
FIG. 6 shows a formation of a high-frequency output signal to be transmitted RF-OUT a simplified diagram.
In this case a complex baseband input signal BB-IN1 is routed to a baseband clipping unit BB-CL. A clipped output signal CL-OUT formed in such a case is fed on the input side to an interpolation unit BB-INT. The interpolation unit BB-INT is for example embodied as a “Root Raised Cosine, RRC” filter and is used for bandwidth limitation.
An output signal RRC-OUT formed by the interpolation unit BB-INT is fed to a mixer M1 which samples the fed-in signal at a high sample rate and converts it into an intermediate frequency position ZF, so that an output signal ZF-OUT1 is formed. A frequency offset is formed by the mixer M1 in the output signal ZF-OUT1.
In the same manner further complex baseband input signals BB-IN2 to BB-INx are converted in further x−1 parallel branches into intermediate frequency signals ZF-OUT2 to ZF-OUTx.
Subsequently the intermediate frequency signals ZF-OUT1 to ZF-OUTx are overlaid additively in a summation unit SUM and combined into an intermediate frequency summation signal ZF-SUM.
The summation signal ZF-SUM is fed to a unit ZF-INT and there is both interpolated (“interpolation”) and also fully clipped in the ZF position (“IF clipping”), so that an intermediate-frequency output signal ZF-INT-OUT is formed.
The output signal ZF-INT-OUT is fed to a unit INT/MOD which both interpolates it (“interpolation”) and also converts it by modulation into a carrier frequency position RF so that a carrier frequency output signal RF-OUT is formed.
With the clipping method in the baseband by the base band clipping unit BB-CL, a clipped output signal CL-OUT is formed, which however because of the downstream RRC filter does not influence any frequency bands adjacent to the useful signal band.
With the clipping method in the intermediate frequency range ZF which is undertaken using the unit ZF-INT, maximum signal amplitudes which arise can cause additional faults in the adjacent frequency bands.
So that this does not produce any impermissible deterioration of the adjacent channel leakage ratio, ACLR, these faults must be suppressed in the adjacent frequency bands with the appropriate filters. these filters advantageously operate on the analog high-frequency signal.
With what is known as the “rectangular” clipping method inphase component and quadrature component of the complex signal are limited independently of each other.
With what is known as the “circular” clipping method an amount of a complex signal in the baseband does not exceed a predetermined maximum signal value.
FIG. 7 shows a known formation of a carrier-frequency output signal for a radio communication system with multicarriers in a simplified presentation.
An OFDM radio communication system is considered here for example. In this case complex baseband input signals
BB-CIS are fed to a unit IFFT with the aid of which an inverse Fast Fourier transformation is performed.
The complex baseband input signals BB-CIS are transformed into a complex output signal IFFT-OUT. Subsequently guard times are inserted by the unit Tg and a complex baseband input signal BB-IN1 is formed. The guard times are inserted for suppression of the intersymbol interference between the symbols.
With reference to FIG. 6 a carrier-frequency output signal RF-OUT is then formed in the corresponding manner.